This would be an optional game mechanic to make the game generally more forgiving:
Basically, a Merc's health can go no lower than 5hp by taking damage from weapons or whatever. However, when they do get to that 5hp mark, they're incapacitated and will bleed at a rate of 1hp per turn.
The intent would be to make it so you always have the chance to save (a) merc(s) who would otherwise have been killed outright, while still carrying the risk that they will die. It seems that the possibility of a merc getting one-shotted with no opportunity to react puts a lot of people off ironman mode or otherwise sucking it up when it goes bad, this may help alleviate that.

I believe that this topic has been discussed several times already and the change request was denied for several reasons. One of the reasons was that mercs, soldiers, NPCs use the same code for calculating damage and finally death. There are thresholds for certain stuff that apply to any being in the game. All of this should be changed just because someone doesn't want to load the last savegame? I doubt that any coder would be willing to rework the whole code, playtest it afterwards and fix all the bugs that he implemented in the process.
Also it makes absolutely no sense to give mercs an advantage like this. We can kill the enemy with one shot and our opponents can't? Why?

Players can already play soft ironman mode which allows to save anytime except during turn based combat. Got one of your mercs killed? Just go back to the last savegame if you can't live with it.

I believe that this topic has been discussed several times already and the change request was denied for several reasons. One of the reasons was that mercs, soldiers, NPCs use the same code for calculating damage and finally death. There are thresholds for certain stuff that apply to any being in the game. All of this should be changed just because someone doesn't want to load the last savegame? I doubt that any coder would be willing to rework the whole code, playtest it afterwards and fix all the bugs that he implemented in the process.
Also it makes absolutely no sense to give mercs an advantage like this. We can kill the enemy with one shot and our opponents can't? Why?

Players can already play soft ironman mode which allows to save anytime except during turn based combat. Got one of your mercs killed? Just go back to the last savegame if you can't live with it.

I don't disagree, didn't realise this had been suggested before. Is there a proper ironman mode?